```python

import datetime as dt

def greet_based_on_time():

now = dt.datetime.now().time()

morning_threshold = dt.time(10, 0)

evening_threshold = dt.time(20, 0)

if now < morning_threshold:

print('GM')

elif now >= evening_threshold:

print('GN')

else:

raise Exception('Invalid time for greeting')

try:

greet_based_on_time()

except Exception as e:

print(e)

```